Image interpolation apparatus and method

ABSTRACT

The present invention is an interpolation device and method. According to the present invention, a plurality of pixels close to a pixel to be interpolated are sequentially set to a central pixel. When a pixel difference closest to a threshold value among pixel differences between the central pixel and a plurality of pixels around the central pixel belongs to a quasi-edge decision range, an edge interpolation and a bilinear interpolation are mixed to interpolate the pixel to be interpolated. As a result, it is possible to prevent reduction in image quality due to an edge verdict caused by unstable input image signal and perform a stable interpolation operation.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/987,615, filed on Nov. 12, 2004, which relies for priority uponKorean Patent Application No. 2003-80303, filed on Nov. 13, 2003, thecontents of which are incorporated herein in their entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of this Invention

The invention relates to image signal processor systems and moreparticularly, to an image signal processor system for interpolating withedge information in order to extend an image signal.

2. Description of Prior Art

Generally, interpolations have been used to perform a zoom function forextending image signals. Image signals are classified as signalsconstructed with only natural image and image signals being mixed withthe natural image and text or graphics.

Recently, as multimedia personal computers (PC) have been widely used,image processes are performed with respect to complex images such as theimage signal mixed with the natural image and text or graphics. In orderto extend images, linear interpolation methods have been generally usedin recent years. However, in the case in which only a linearinterpolation method is used, there is a problem of weak edgeinformation. Accordingly, it is necessary to maintain the sharpness ofedges in a region where edge information such as graphics or text is amain factor. For this, edge information for performing a suitableprocess by segmenting graphics or text regions and natural image regionis needed.

Linear interpolation has advantages and disadvantages in an extendedimage. Noise is reduced in the natural image region, but edgeinformation is not efficiently expressed in the graphic region. Thereason for this is that the linear interpolation method does notconsider edge components.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the more particular description of apreferred embodiment of the invention, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic block diagram of an image signal interpolationdevice according to a preferred embodiment of the present invention.

FIG. 2 illustrates operation of an edge detector shown in FIG. 1.

FIG. 3 illustrates a general standard for determining whether a centralpixel is an edge pixel according to calculated pixel differences.

FIG. 4 illustrates an unstable image inputted to the image signalinterpolation device of the invention.

FIG. 5 illustrates a standard of an edge detector for determiningwhether a pixel to be interpolated X and closing pixels PF, PG, PJ andPK are edge pixels.

FIG. 6 illustrates a pattern of an edge between a central pixel andeight closing pixels, and a transformed edge pattern.

FIG. 7 illustrates the pixel X to be interpolated and four closingpixels PNW, PNE, PSW and PSE.

FIG. 8 illustrates a central edge model and a rotation angle.

FIG. 9 illustrates a coordinate transformation.

FIG. 10 illustrates a calculation process of edge interpolationcoefficients with respect to the central edge model.

FIG. 11 illustrates a 4-tap linear interpolation method.

FIG. 12 is a flowchart showing an interpolation method according to apreferred embodiment of the present invention when the pixel value ofthe pixel to be interpolated X and four closing pixels are trembled onthe basis of a predetermined level.

SUMMARY OF THE INVENTION

One feature of the present invention is to provide an imageinterpolation device for detecting an edge of a graphic or a text tointerpolate it.

Another feature of the present invention is to provide an imageinterpolation device capable of stably interpolating it from an unstableinput image signal.

According to one feature of the present invention, an interpolationmethod comprises: sequentially setting a plurality of pixels close to apixel to be interpolated to a central pixel; calculating pixeldifferences between the central pixel and a plurality of pixels aroundthe central pixel; based on the pixel differences, determining whetherthe central pixel is in a quasi-edge decision range, the central pixelbeing in the quasi-edge decision range when the pixel difference closestto a predetermined pixel difference threshold indicates that the centralpixel is in the quasi-edge decision range; and, if the central pixel isin the quasi-edge decision range, performing an interpolation, theinterpolation comprising a mix of an edge interpolation and a bilinearinterpolation.

In a preferred embodiment, in performing the interpolation, the mixingratio of the edge interpolation and the bilinear interpolation isdetermined according to the difference value between a pixel differenceclosest to the threshold value among the pixel differences and thethreshold value.

In one embodiment, the threshold value is one of a minimum thresholdvalue and a maximum threshold value.

In one embodiment, the quasi-edge decision range is larger than theminimum lowest limit value and smaller than the minimum upper value atthe center of the minimum threshold value.

In one embodiment, when the pixel difference closest to the minimumthreshold value among the pixel differences is larger than the minimumlowest limit value and smaller than the minimum upper value, the mixingratio is

(TH_L+LU−dm)*m

where, TH_L represents the minimum threshold value, LU represents theminimum upper limit value, dm represents a pixel difference closest tothe minimum threshold value, and m represents a roll-off of the upperlimit value and the lowest limit value.

In one embodiment, the quasi-edge decision range is larger than themaximum lowest limit value and smaller than maximum upper limit value atthe center of the maximum threshold value.

In one embodiment, when the pixel difference closest to the maximumthreshold value among the pixel differences is larger than the maximumlowest limit value and smaller than the maximum upper value, the edgeinterpolation ratio is

(TH_H+HL−dm)*m

where, TH_H represents the maximum threshold value, HL represents thelowest limit value, dm represents a pixel difference closest to themaximum threshold value, and m represents a roll-off of the upper limitvalue and the lowest limit value.

In a preferred embodiment, the edge interpolation calculates a pixelvalue of the pixel to be interpolated from a pixel value of four pixelsclose to the pixel to be interpolated and edge interpolationcoefficients corresponding to the four close pixels.

In this embodiment, the pixel value of the interpolation pixel isPNW*(W_NW)+PNE*(W_NE)+(W_SW)+PSE*(W_SE)

where, PNW, PNE, PSW and PSE are four pixels close to the pixel to beinterpolated, and W_NW, W_NE, W_SW and W_SE are interpolationcoefficients respectively corresponding to the four close pixels.

In this embodiment, the interpolation coefficients W_NW, W_NE, W_SW andW_SE are

W _(—) NW=t(WNWE)+(1−t)WNWB

W _(—) NE=t(WNEE)+(1−t)WNEB

W _(—) SW=t(WSWE)+(1−t)WSWB

W _(—) SE=t(WSEE)+(1−t)WSEB

where, t represents the mixing ratio, WNWE, WNEE, WSWE and WSEE are edgeinterpolation coefficients, WNWB, WNEB, WSWB and WSEB are bilinearinterpolation coefficients, and the bilinear interpolation coefficientsare

WNWB=(1−KX)*(1−KY)

WNEB=KX*(1−KY)

WSWB=(1−KX)*KY

WSEB=KX*KY

where, KX and KY are respectively a position of a pixel to beinterpolated.

According to another feature of the present invention, an interpolationmethod comprises sequentially setting a plurality of pixels close to apixel to be interpolated to a central pixel. Pixel differences betweenthe central pixel and a plurality of pixels around the central pixel arecalculated. An interpolation ratio is calculated when a pixel defferenceclosest to a threshold value among the pixel defferences belongs to aquasi-edge decision range. An interpolation coefficient is calculatedbased on the interpolation ratio. The pixel to be interpolated isinterpolated based on the interpolation coefficient when an edge existsbetween the interpolation pixel and the close pixels.

In a preferred embodiment, four pixels around the pixel to beinterpolated are sequentially set to the central pixel.

In a preferred embodiment, pixel differences between the central pixeland eight pixels around the central pixel are respectively calculated.

In a preferred embodiment, the threshold value is the maximum thresholdvalue or minimum threshold value.

In this embodiment, when the threshold value is the minimum thresholdvalue, the first quasi-edge verdict range is larger than the minimumlowest limit value and smaller than the minimum upper limit value at thecenter of the minimum threshold value. When the threshold value is themaximum threshold value, the second quasi-edge decision range is largerthan the maximum lowest limit value and smaller than the maximum upperlimit value at the center of the maximum threshold value.

In this embodiment, when the pixel difference closest to the minimumthreshold value among the pixel differences is larger than the minimumlowest limit value and smaller than the minimum upper value, the mixingratio is

(TH_L+LU−dm)*m

where, TH_L represents the minimum threshold value, LU represents theminimum upper limit value, dm represents a pixel difference closest tothe minimum threshold value, and m represents a roll-off of the upperlimit value and the lowest limit value.

In this embodiment, when the pixel difference closest to the minimumthreshold value among the pixel differences is larger than the minimumlowest limit value and smaller than the maximum upper value, the edgeinterpolation ratio is

(TH_H+HL−dm)*m

where, TH_H represents the maximum threshold value, HL represents thelowest limit value, dm represents a pixel difference closest to themaximum threshold value, and m represents a roll-off of the upper limitvalue and the lowest limit value.

In this embodiment, the pixel value of the pixel to be interpolated isPNW*(W_NW)+PNE*(W_NE)+PSW*(W_SW)+PSE*(W_SE)

where, PNW, PNE, PSW and PSE are four pixels close to the pixel to beinterpolated, and W_NW, W_NE, W_SW and W_SE are interpolationcoefficients, which are calculated in the step of calculating theinterpolation coefficient.

In this embodiment, the interpolation coefficients W_NW, W_NE, W_SW andW_SE are

W _(—) NW=t(WNWE)+(1−t)WNWB

W _(—) NE=t(WNEE)+(1−t)WNEB

W _(—) SW=t(WSWE)+(1−t)WSWB

W _(—) SE=t(WSEE)+(1−t)WSEB

where, t represents a mixing ratio calculated in the step of calculatingthe mixing ratio, WNWE, WNEE, WSWE and WSEE are edge interpolationcoefficients calculated from a position of a coordinate transformationof the pixel to be interpolated, WNWB, WNEB, WSWB and WSEB are bilinearinterpolation coefficients, and the bilinear interpolation coefficientsare

WNWB=(1−KX)*(1−KY)

WNEB=KX*(1−KY)

WSWB=(1−KX)*KY

WSEB=KX*KY

where, KX and KY are respectively a position of a pixel to beinterpolated.

In this embodiment, the mixing ratio (t) is 1 in case that a pixeldifference closest to the minimum threshold value among the pixeldifferences is the same as or smaller than the minimum lowest limitvalue. The mixing ratio (t) is 0 in case that a pixel difference closestto the minimum threshold value among the pixel differences is smallerthan the maximum lowest limit value.

In this embodiment, in case that a pixel difference closest to themaximum threshold value among the pixel differences is larger than orthe same as the maximum upper limit value, the mixing ratio (t) is 1. Incase that a pixel difference closest to the maximum threshold valueamong the pixel differences is larger than the maximum upper limitvalue, the mixing ratio (t) is 0.

In a preferred embodiment, a step of linearly interpolating is furtherincluded when an edge does not exist between the pixel to beinterpolated and the close pixel.

According to still another feature of the present invention, aninterpolation device comprises an edge detector for sequentially settinga plurality of pixels close to a pixel to be interpolated andcalculating pixel differences between a central pixel and a plurality ofpixels around the central pixel to detect an edge between the pixel tobe interpolated and the close pixels; an interpolation mix ratiocalculator for calculating an interpolation mix ratio when a pixeldifference closest to a threshold value among the pixel differencesbelongs to a quasi-edge decision range; and a non-linear interpolatorfor interpolating the pixel to be interpolated by mixing an edgeinterpolation and a bilinear interpolation based on the interpolationratio.

In a preferred embodiment, the edge detector sequentially sets fourpixels close to the pixel to be interpolated to a central pixel andcalculates pixel differences between the central pixel and eight pixelsaround the central pixel to detect an edge between the pixel to beinterpolated and the close pixels.

In a preferred embodiment, the threshold value is the minimum thresholdvalue or the maximum threshold value.

In this embodiment, the interpolation ratio calculator calculates aninterpolation ratio when a pixel difference closest to the minimumthreshold value among the pixel differences belongs to a firstquasi-edge decision range. In addition, the interpolation ratiocalculator calculates the interpolation ratio when a pixel differenceclosest to the maximum threshold value among the pixel differencesbelongs to a second quasi-edge decision range.

The first quasi-edge decision range is larger than the minimum lowestlimit value and smaller than the minimum upper limit value at the centerof the minimum threshold value. The second quasi-edge decision range islarger than maximum lowest limit value and smaller than the maximumupper limit value at the center of the maximum threshold value.

In a preferred embodiment, a step of linearly interpolating is furtherincluded when an edge does not exists between the interpolation pixeland the close pixel.

According to still another feature of the present invention, theinterpolation device comprises an edge detector sequentially settingfour pixels close to a pixel to be interpolated and calculating pixeldifferences between a central pixel and eight pixels around the centralpixel to detect a temporary edge between the pixel to be interpolatedand the close pixels; an edge direction modifier for transforming atemporary edge detected from the edge detector to generate predeterminedtransformed edge information; an interpolation mix ratio calculator forcalculating an interpolation ratio according to a relation of the pixeldifferences, and the minimum threshold value and the maximum thresholdvalue; a non-linear interpolation coefficient generator for generating anon-linear interpolation coefficient from edge information from the edgedirection modifier and the interpolation mix ratio calculator andoutputting an edge detection signal showing whether an edge is betweenthe pixel to be interpolated and the close pixels; a linear interpolatorfor linearly interpolating the pixel to be interpolated; a non-linearinterpolator for non-linearly interpolating the pixel to be interpolatedreferring to the non-linear interpolation coefficient; and a dataselector for outputting any one pixel value interpolated in the linearinterpolator and the non-linear interpolator as a pixel value of thepixel to be interpolated in response to the edge detection signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an image signal interpolation device according to apreferred embodiment of the present invention. Referring to FIG. 1, theimage signal interpolation device 100 includes an edge detector and anon-linear interpolation coefficient generator 110, a linearinterpolator 120, a non-linear interpolator 130 and a data selector 140.The edge detector and non-linear interpolation coefficient generator 110include an edge detector 111, an edge direction modifier 112, anon-linear interpolation coefficient generator 113, and an interpolationmix ratio calculator 114. The function and operation of the respectivecomponent of the image signal interpolation device 100 will be morefully described hereinafter.

FIG. 2 shows operation the edge detector 111 shown in FIG. 1.

Referring to FIG. 2, the edge detector 111 detects an edge between apixel X to be interpolated (hereinafter inclusively referred to as “aninterpolation pixel”) and neighboring pixels PF, PG, PJ and PK. In orderto detect the edge, the edge detector 111 determines whether theinterpolation pixel X and the neighboring pixels PF, PG, PJ and PK arean edge. The determination method is described as follows.

In advance, the edge detector 111 sequentially sets the neighboringpixels PF, PG, PJ and PK to a central pixel. When the neighboring pixelsPF are the central pixels, the edge detector 111 calculates thedifference of each of the pixel PF, the neighboring eight pixels PA, PB,PC, PE, PG, PI, PJ and PK. These differences may be calculated by one ofmathematical equations 1, 2 and 3.

d(i,j)(k,l)=|R(i,j)−R(k,l)|+|G(i,j)−G(k,l)|+|B(i,j)−B(k,l)|  [Mathematicalequation 1]

d(i,j)(k,l)={R(i,j)−R(k,l)}² +{G(i,j)−G(k,l)}²+{B(i,j)−B(k,l)}²  [Mathematical equation 2]

d(i,j)(k,l)=max{|R(i,j)−R(k,l)|,|G(i,j)−G(k,l)|,|B(i,j)−B(k,l)|}  [Mathematicalequation 3]

In the above mathematical equations 1 to 3, (i, j) represents a positionof the central pixel, (k, l) represents a position of the neighboringpixels, R represents Red, G represents Green, and B represents Blue.

The edge detector 111 determines whether the neighboring pixels PF, PG,PJ and PK are edge pixels or not by comparing the maximum thresholdvalue TH_H and the minimum threshold value TH_L with pixel differencescalculated by one of equations 1 to 3. FIG. 3 shows a general standardfor determining whether a central pixel is an edge pixel according tocalculated pixel differences. In general, in case that all calculatedpixel differences are larger or smaller than the maximum threshold valueTH_H and minimum threshold value TH_L, the central pixel is determinedto be an edge pixel. Otherwise, the central pixel is not determined tobe the edge pixel. The reason for this is that only if the central pixeland eight neighboring pixels have remarkable difference or almost samevalue with respect to the central pixel, is it possible to determine thecentral pixel to be the edge pixel. In addition, under thisdetermination condition, it is possible to restrain the influence due toa noise component in an image and edge detection in most of naturalimage changed successively and slowly.

As shown in FIG. 4, when an image inputted to the image signalinterpolation device is stable, and pixel differences between eightpixels neighboring the central pixel is changed around the minimumthreshold value or the maximum threshold value, an edge pixeldetermination with respect to the neighboring pixels PF, PG, PJ and PKbecomes unstable. That is, the neighboring pixels are alternatelydetermined to be an edge pixel and a non-edge pixel. Resultantly, anedge interpolation and linear interpolation are discontinuouslyperformed with respect to the interpolation pixel, so that pixel flickeris generated on a screen.

Accordingly, when the pixel difference for determining whether the pixeldifference is about the neighboring pixels or not is around the minimumthreshold value or the maximum threshold value, image quality isimproved by performing an interpolation through a new method inaccordance with the invention.

FIG. 5 shows a standard of an edge detector 111 determining whetherpixels PF, PG, PJ and PK neighboring the interpolation pixel X are edgepixels or not. As previously described, the neighboring pixels PF, PG,PJ and PK are sequentially set to the central pixel, and the differencebetween eight pixels neighboring the central pixel is calculated. Incase that all calculated pixel differences are smaller than the minimumlowest limit value LL or larger than the maximum upper limit value HH,the central pixel is determined to be an edge pixel. In case that apixel difference closest to the minimum threshold value TH_L among thecalculated pixel differences belongs to a first quasi-edge decisionrange Q1, or a pixel difference closest to the maximum threshold valueTH_H belongs to a second quasi-edge decision range Q2, the central pixelis determined to be a quasi-edge pixel. The first quasi-edge decisionrange Q I is larger than the minimum lowest limit value LL and smallerthan the minimum upper value LU at the center of the minimum thresholdvalue TH_L. The second quasi-edge decision range Q2 is larger than themaximum lowest limit value LU and smaller than the maximum upper valueHU at the center of the maximum threshold value TH_H.

A method for determining whether the central pixel is a quasi-edge pixelis more fully described hereinafter. In advance, the differences betweeneight pixels neighboring the central pixel are calculated. A pixeldifference closest to the minimum threshold value TH_L or the maximumthreshold value TH_H among the calculated differences is searched. Incase that a pixel difference closest to the minimum threshold value TH_Lor the maximum threshold value TH_H is between the minimum lowest limitvalue LL and the minimum upper value LU (Q1), or the maximum lowestvalue HL and the maximum upper value HU (Q2), the central pixel isdetermined to be a quasi-edge pixel. Where, LU−LL=HU−HL.

The pixel difference dm closest to the minimum threshold value TH_L orthe maximum threshold value TH_H among the calculated differences isprovided to the interpolation mix ratio calculator 114.

The interpolation mix ratio calculator 114 calculates an interpolationratio in response to the minimum pixel difference dm. In advance, whenthe pixel difference dm is close to the maximum threshold value TH_H,the interpolation mix ratio calculator 114 calculates the interpolationratio t according to the following mathematical equation 4.

if (dm<HL) t=0

else if (dm≧HH) t=1

else t=(dm−HL)*m  [Mathematical Equation 4]

When the pixel difference dm is close to the minimum threshold valueTH_L, the interpolation mix ratio calculator 114 calculates theinterpolation ratio t according to the following mathematical equation5.

if (dm<LL) t=0

else if (dm≦HH) t=1

else t=(LU−dm)*m  [Mathematical Equation 5]

In the above mathematical equations 4 and 5, m represents roll-off ofthe maximum upper value and the maximum lowest value (1/(HU−HL)) or ofthe minimum upper value and minimum lowest value (1/(LU−LL)). Where, m'smaximum value is 1.

A method for determining whether an interpolation pixel and neighboringpixels are an edge is disclosed in Korean Laid Open Publication No.2000-59958 entitled, “EDGE DETECTION METHOD IN MIX IMAGE OF GRAPHIC ANDNATURAL IMAGE AND EDGE DETECTION DEVICE,” the contents of which areincorporated herein in their entirety be reference.

The edge direction modifier 112 generates predetermined modified edgeinformation MOD_EDG diagonal components on the basis of a pivot of fourpixels (top, bottom, right and left) by modifying temporary edgedetected from the edge detector 111. The modified edge informationMOD_EDG may be classified into a border edge component and a center edgecomponent. The center edge component may be expressed in eightdirections including internal horizontal and vertical components, anddiagonal components on the basis of the pivot. FIG. 6 shows variouspatterns to be referred to an edge modification process in the edgedirection modifier 112. In particular, resultant patterns of edgemodification with respect to 3*3 blocks are shown in FIG. 6. FIG. 6shows edge patterns detected from the edge detector 111, and B showsmodified edge patterns by the edge direction modifier 112. As shown inFIG. 6, most of the edge patterns are modified to diagonal edges. Theedge information MOD_EDG is information with respect to the pattern B.

The detailed function of the edge direction modifier 112 is disclosed inKorean Laid Open Publication No. 2002-4246 entitled, “TWO DIMENSIONALMIX INTERPOLATION DEVICE FOR IMPROVING EDGE AND METHOD FOR THEREOF,” thecontents of which are incorporated herein in their entirety byreference. Therefore, the description of the edge direction modifier 112is not repeated herein.

If all modified edge information MOD_EDG is received when fourneighboring pixels PF, PF, PJ and PK are respectively set to the centralpixel from the edge direction modifier 112, the non-linear coefficientgenerator 113 generates the edge information MOD_EDG, non-linearinterpolation coefficients W_NW, W_NE, W_SW and W_SE, and a selectionsignal SEL.

The non-linear interpolation coefficients are values multiplied by theinterpolation pixel X and four neighboring pixels. FIG. 7 shows theinterpolation pixel X and four neighboring pixels PNW, PNE, PSW and PSE.The pixel PNW is located in the northwest, the pixel PNE is located inthe northeast, the pixel PSW is located in the southwest, and the pixelPSE is located in the southeast at the center of a position (KX, KY) ofthe interpolation pixel X. In FIG. 2, the pixels PF, PG, PJ and PKneighboring the interpolation pixel X are equivalent to PNW, PNE, PSWand PSE, respectively. The pixel value of the interpolation pixel X iscalculated by PNW*(W_NW)+PNE*(W_NE)+PSW*(W_SW)+PSE(W_SE). The non-linearinterpolation coefficients W_NW, W_NE, W_SW and W_SE due to thequasi-edge decision are calculated by mathematical equation 6.

W _(—) NW=t(WNWE)+(1−t)WNWB

W _(—) NE=t(WNEE)+(1−t)WNEB

W _(—) SW=t(WSWE)+(1−t)WSWB

W _(—) SE=t(WSEE)+(1−t)WSEB  [Mathematical Equation 6]

In the mathematical equation 6, WNWE, WNEE, WSWE and WSEE are edgeinterpolation coefficients calculated from a position (CX, CY)transformed by coordinate transformation of a position (KX, KY) of theinterpolation pixel X, and WNWB, WNEB, WSWB and WSEB are bilinearinterpolation coefficients calculated from the position (KX, KY) of theinterpolation pixel X. The bilinear interpolation coefficients WNWB,WNEB, WSWB and WSEB are obtained by mathematical equation 7.

WNWB=(1−KX)*(1−KY)

WNEB=KX*(1−KY)

WSWB=(1−KX)*KY

WSEB=KX*KY  [Mathematical Equation 7]

The non-linear coefficient generator 113 receives the transformed edgeinformation MOD_EDG to set a proto-type center edge. At this time, theproto-type center edge may have six shapes. In addition, the non-linearcoefficient generator 113 determines a rotation angle with respect tothe proto-type center edge.

FIG. 8 illustrates the proto-type center edge and the rotation angle.The prototype center edge is classified into six shapes such as corner,perpendicular, full line, cross bar, half line and diagonal.

Referring to (a) to (f) shown in FIG. 8, single, twin and periodic edgetypes are determined at the center of six edge shapes according towhether they are boundary edge types.

FIG. 9 illustrates a coordinate transformation. In FIG. 9, (a)represents a coordinate (x, y) with respect to an interpolation position(KX, KY) before transformation, and (b) represents a coordinate (x′, y′)of an interpolation position (CX, CY) with respect to a rotation angleROT_ANG after transformation. If the rotation angle by the proto-typecentral edge is determined, the interpolation position is transformedtogether with coordinate axes.

A generation process of the non-linear interpolation coefficients WNW,WNE, WSE and WSE of the interpolation coefficient generator 113 shown inFIG. 1 is shown in (a) to (f) of FIG. 10.

In mathematical equations 4 to 5, if t=0, the non-linear interpolationcoefficients W_NW, W_NE, W_SW and W_SE of FIG. 6 are the same as edgeinterpolation coefficients WNWE, WNEE, WSWE and WSEE. In addition, incase 0<t<1, the sum of the edge interpolation coefficients WNWE, WNEE,WSWE and WSEE according to an interpolation ratio t and theinterpolation coefficients WNWB, WNEB, WSWB and WSEB is determined tothe non-linear interpolation coefficients W_NW, W_NE, W_SW and W_SE. Thenon-linear interpolation coefficients W_NW, W_NE, W_SW and W_SE areprovided to the non-linear interpolator 130. In the meanwhile, thenon-linear interpolation coefficient generator 113 determines whether anedge is between the interpolation pixel X and the neighboring pixels PF,PG, PJ and PK from the transformed edge information MOD_EDG. If there isthe edge, the non-linear interpolation coefficient generator 113activates the edge detection signal DET_EDG.

Referring to FIG. 1 again, the linear interpolation 120 linearlyinterpolates the interpolation pixel X from an inputted image signal.FIG. 11 shows a linear interpolation method. Referring to FIG. 11,sixteen pixels (4*4) PA˜PP are requested to a 4-tab linearinterpolation. In advance, the linear interpolation is verticallyperformed to generate the interpolated pixels IA, IB, IC and ID. And, itis possible to obtain an interpolation value of a pixel X to beinterpolated by performing a horizontal interpolation from theinterpolated pixels IA, IB, IC and ID.

As shown in FIG. 2, the non-linear interpolator 130 obtains a pixelvalue X of the interpolation pixel by respectively multiplying thenon-linear coefficients four neighboring pixels PF, PG, PJ and PK by thenon-linear coefficients W_NW, W NE, W_SW and W_SE.

The selector 140 outputs one of pixel values from the linearinterpolator 120 and the non-linear interpolator 130 as a pixel value ofthe interpolation pixel X in response to the edge detection signalDET_EDG. That is, when the edge detection signal DET_EDG is in aninactivated -state, meaning an edge non-detection, a pixel value fromthe linear interpolator 120 is outputted as an interpolation pixel X.When the edge detection signal DET_EDG is in an activated state, meaningan edge detection, a pixel value from the non-linear interpolator 130 isoutputted as an interpolation signal.

FIG. 12 is a flowchart showing an interpolation method according to apreferred embodiment of the present invention when the value of theinterpolation pixel X and four closing pixels are trembled on the basisof a predetermined level.

In advance, as shown in FIG. 2, the interpolation pixel X and fourneighboring pixels PF, PG, PJ and PK are sequentially set to a centralpixel in step S300. In step S301, a pixel difference of a central pixeland eight pixels around the central pixel is calculated. That is, whenthe central pixel is PF, eight pixels are PA, PB, PC, PE, PG, PI, PJ andPK. In step S302, when a pixel difference closest to the minimumthreshold among pixel differences belongs to a second quasi-edgedecision range Q2, an interpolation ratio t is calculated. Theinterpolation ratio t is a mix ratio of the edge interpolation andbilinear interpolation. In step S303, the interpolation coefficientsW_NW, W_NE, W_SW and W-SE are calculated based on the interpolationratio t. In step S304, when an edge exists between the interpolationpixel X and the neighboring pixels, the non-linear interpolation isperformed based on the interpolation coefficient.

According to the present invention, when the pixel value of theinterpolation pixel X and four closing pixels are trembled on the basisof a predetermined level, the interpolation coefficient is calculated,and the interpolation is performed based on the interpolation ratio t.Therefore, the interpolation pixel and neighboring pixels arediscontinuously determined to be the edge pixel and non-edge pixel, sothat the interpolation pixel is discontinuously interpolated by thelinear and non-linear interpolation methods. Resultalty, it is possibleto prevent image quality from being degraded.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. An interpolation device, comprising: an edge detector sequentiallysetting four pixels close to a pixel to be interpolated and calculatingpixel differences between a central pixel and eight pixels around thecentral pixel to detect a temporary edge between the interpolation pixeland the close pixels; an edge direction modifier for transforming atemporary edge detected from the edge detector to generate apredetermined transformed edge information; an interpolation mix ratiocalculator for calculating an interpolation ratio according to arelation of the pixel differences, and the minimum threshold value andthe maximum threshold value; a non-linear interpolation coefficientgenerator for generating a non-linear interpolation coefficient fromedge information from the edge direction modifier and the interpolationmix ratio calculator and outputting an edge detection signal showingwhether an edge is between the interpolation pixel and the close pixels;a linear interpolator for linearly interpolating the interpolationpixel; a non-linear interpolator for non-linearly interpolating theinterpolation pixel referring to the non-linear interpolationcoefficient; and a data selector for outputting any one pixel valueinterpolated in the linear interpolator and the non-linear interpolatoras a pixel value of the pixel to be interpolated in response to the edgedetection signal.
 2. The interpolation device of claim 1, wherein whenthe pixel difference closest to the minimum threshold value among thepixel differences is larger than the minimum lowest limit value andsmaller than the minimum upper value, the edge interpolation ratio is(TH_L+LU−dm)*m where, TH_L represents the minimum threshold value, LUrepresents the minimum upper limit value, dm represents a pixeldifference closest to the minimum threshold value, and m represents aroll-off of the upper limit value and the lowest limit value.
 3. Theinterpolation device of claim 1, wherein when the pixel differenceclosest to the maximum threshold value among the pixel differences islarger than the maximum lowest limit value and smaller than the maximumupper value, the edge interpolation ratio is(TH_H+HL−dm)*m where, TH_H represents the maximum threshold value, HLrepresents the lowest limit value, dm represents a pixel differenceclosest to the maximum threshold value, and m represents a roll-off ofthe upper limit value and the lowest limit value.
 4. The interpolationdevice of claim 1, wherein the non-linear interpolator outputsPNW*(W_NW)+PNE*(W_NE)+PSW*(W_SW)+PSE*(W_SE) to an interpolation value ofthe pixel to be interpolated where, PNW, PNE, PSW and PSE are fourpixels close to the pixel to be interpolated, and W_NW, W_NE, W_SW andW_SE are interpolation coefficients, which are calculated in the step ofcalculating the interpolation coefficient.
 5. The interpolation deviceof claim 4, wherein the interpolation coefficients W_NW, W_NE, W_SW andW_SE areW _(—) NW=t(WNWE)+(1−t)WNWBW _(—) NE=t(WNEE)+(1−t)WNEBW _(—) SW=t(WSWE)+(1−t)WSWBW _(—) SE=t(WSEE)+(1−t)WSEB where, t represents a mixing ratiocalculated in the step of calculating the mixing ratio, WNWE, WNEE, WSWEand WSEE are edge interpolation coefficients calculated from a positionof a coordinate transformation of the pixel to be interpolated, WNWB,WNEB, WSWB and WSEB are bilinear interpolation coefficients, and thebilinear interpolation coefficients areWNWB=(1−KX)*(1−KY)WNEB=KX*(1−KY)WSWB=(1−KX)*KYWSEB=KX*KY where, KX and KY are respectively a position of a pixel to beinterpolated.
 6. The interpolation device of claim 5, wherein the mixingratio (t) is 1 in the case that a pixel difference closest to theminimum threshold value among the pixel differences is the same as orsmaller than the minimum lowest limit value, and wherein the mixingratio (t) is 0 in the case that a pixel difference closest to theminimum threshold value among the pixel differences is smaller than themaximum lowest limit value.
 7. The interpolation device of claim 6,wherein in the case that a pixel difference closest to the maximumthreshold value among the pixel differences is larger than or the sameas the maximum upper limit value, the mixing ratio (t) is 1, and whereinin case that a pixel difference closest to the maximum threshold valueamong the pixel differences is larger than the maximum upper limitvalue, the mixing ratio (t) is 0.